1. Field of the Invention
The present invention relates to a back light unit, and more particularly, to a prism sheet capable of achieving an enhancement in light efficiency and viewing angle characteristics and a backlight unit using the prism sheet.
2. Discussion of the Related Art
Recently, various flat panel display devices have been developed which can eliminate disadvantages of cathode ray tubes (CRTs) caused by the bulky and heavy structures thereof. Such flat panel display devices include a liquid crystal display (LCD), a field emission display (FED), a plasma display panel (PDP), and a light emitting display (LED).
Of such flat panel display devices, the LCD displays a desired image by adjusting the transmission level of light beams irradiated from a backlight unit using a liquid crystal panel including a plurality of liquid crystal cells and a plurality of control switches to apply video signals to the liquid crystal cells.
FIG. 1 is a schematic view illustrating a conventional backlight unit.
Referring to FIG. 1, the conventional backlight unit includes a lamp 10 that emits light, a light guide plate 20 that receives the light emitted from the lamp 10, and emits the light across the surface area, and a lamp housing 12 that encloses an incidence face 22 of the light guide plate 20 and the lamp 10. The conventional backlight unit also includes a reflection plate 30 that is arranged beneath the light guide plate 20, a diffusion sheet 40 that is arranged above the light guide plate 20 to diffuse light emerging from the light guide plate 20, and a prism sheet 50 which adjusts the propagation direction of light emerging from the diffusion sheet 40.
Generally, the lamp 10 is formed of a cold cathode fluorescent lamp. The lamp 10 is turned on by a lamp drive voltage supplied from an inverter not shown, to irradiate light to the incidence face 22 of the light guide plate 20 provided at one side of the light guide plate 20.
The lamp housing 12 is arranged at one side of the light guide plate 20 to enclose the lamp 10 and the incidence face 22 of the light guide plate 20. The lamp housing 12 has on an inner surface a reflection face to reflect the light from the lamp 10 toward the incidence face 22 of the light guide plate 20.
The light guide plate 20 enables the incidence light from the lamp 10 to reach a position spaced apart from the lamp 10 by a great distance, and guides the incidence light toward the diffusion sheet 40. A printed pattern is provided at an inclined lower surface of the light guide plate 20 to form an inclined reflection face, so that light incident to the incidence face 22 is reflected from the inclined reflection face of the light guide plate 20 at a certain angle such that the light travels uniformly toward the diffusion sheet 40.
The reflection plate 30 is arranged beneath the light guide plate 20 in order to reflect light incident thereto from the reflection face of the light guide plate 20 toward the light guide plate 20, and thus, to reduce loss of light.
The diffusion sheet 40 diffuses the light emerging from the light guide plate 20 over the entire region of the diffusion sheet 40, and irradiates the diffused light to the prism sheet 50.
The prism sheet 50 functions to condense the light emerging from the diffusion sheet 40. To this end, the prism sheet 50 includes a condensing film 52 made of polyester (PET), and prism crests 54 formed on an upper surface of the condensing film 52 in the form of stripes, as shown in FIG. 2.
Each prism crest 54 has first and second inclined faces each having a certain inclination from the apex of the prism crest 54. For example, each of the first and second inclined faces is inclined by about 45° from the upper surface of the condensing film 52.
When it is assumed that the condensing film 52 of the prism sheet 50 has a refractive index n1, and the outside of the prism sheet 50 has a refractive index n2, light incident to the condensing film 52 at an angle θ1 is refracted at an angle θ2 at the prism crests 54 in accordance with Snell's Law, as expressed by the following equation 1, and is then emitted to the outside of the prism sheet 50.
                                          n            ⁢                                                  ⁢            1                                n            ⁢                                                  ⁢            2                          =                              sin            ⁢                                                  ⁢            θ1                                sin            ⁢                                                  ⁢            θ2                                              equation        ⁢                                  ⁢        1            
In the above-mentioned conventional backlight unit, light emitted from the lamp 10 advances toward the diffusion sheet 40 arranged above the light guide plate 20 via the light guide plate 20. The light emerging from the light guide plate 20 is diffused by the diffusion sheet 40 over the entire region of the prism sheet 50. The diffused light is condensed while passing through the prism sheet 50, and is then externally emitted.
In the above-mentioned conventional backlight unit, however, the light incident to the prism sheet 50 may be divided into those of three regions, namely, a full reflection region, a condensing region, and a side lobe region, as shown in FIG. 3.
The light in the full reflection region, namely, light A vertically incident to the condensing film 52, is fully reflected by the first and second inclined faces of the prism crests 54, so that the light A is directed back to the light guide plate 20. Thus, the light A is condensed after being recycled.
The light in the condensing region, namely, light B incident to the condensing film 52 at a certain angle, is refracted by the first and second inclined faces of the prism crests 54, so that the light B is condensed.
On the other hand, light in the side lobe region, namely, light C incident to the condensing film 52 at a certain angle, is fully reflected by the first inclined face of the prism crests 54, so that the light C causes a degradation in light efficiency and a degradation in viewing angle characteristics.
For this reason, there are bright regions in the prism sheet 50 at opposite sides of a center line of the prism sheet 50, as shown in FIG. 4.                Thus, the conventional backlight unit has problems caused by the structure of the prism sheet 50, namely, viewing angle asymmetry caused by brightness asymmetry and a degradation in light efficiency caused by side lobes.        